1. Field of the Invention
The present invention relates to a process for producing an optical recording medium to record and read information by means of a laser beam. More particularly, the present invention relates to a process for producing an optical recording medium which possesses superior recording sensitivity and recording stability and is made up of a plastic substrate formed by injection molding, injection-compression molding, compression molding, or a photopolymer process and a thin metal film formed on the substrate.
2. Description of the Background
Optical recording media which record and read information by means of a laser beam are rapidly becoming practical because of recent developments in the fundamental technology of semiconductor lasers, recording materials, and film making processes and also because of the ability of the recording media to record a large amount of information. For optical recording media to be able to record information, a phase change which leads to an optical change should take place in the portions of given medium struck with a laser beam. Bubble making, pitting, and amorphous statecrystalline state transfer are some of the recording methods which have been proposed so far.
One optical recording medium which is known is formed of a plastic substrate, which has a minutely roughened surface structure and a thin metal film formed on its surface. Another optical recording medium which is known is made up of a plastic substrate which have a smooth surface and a thin metal film formed on the substrate. The advantage of the former medium is that recording is possible with low power, because the recording laser beam is absorbed efficiently by the medium. This optical recording medium is disclosed in U.S. Pat. No. 4,616,237.
There still is a demand for a long-life laser source, as well as a demand for a medium which has increase recording capacity. In order to meet this demand, an optical recording medium should be able to record information at a higher speed with a laser beam of lower power. In other words, the optical recording medium should have a higher sensitivity.
Optical recording media normally have a substrate made of glass or plastic. In comparison to glass substrates, plastic substrates are lighter in weight, easier to mold, unbreakable, and more suitable for high-speed recording. On the other hand, plastic substrates have the disadvantage that they are liable to deform because of residual stress imparted at the time of molding of the substrate. Therefore, plastic substrates are not absolutely reliable for use in the manufacture of writable recording media and erasable-rewritable recording media which require long-term reliability and durability. Efforts to eliminate this disadvantage are being made. For example, a heat treatment process is known which removes residual stress as disclosed in, for example, Japanese Patent Laid-Open Nos. 151222/1983, 20719/1986, and 79626/1986.